The present invention relates to a power supply module for supplying power to an electrical power tool that is driven by an electric motor and powered by a rechargeable battery pack.
The battery packs for most, if not all, of the battery-operated power tools available on the market have a limited capacity that restricts the operating time of the tools to about half an hour per pack for normal use. Recharging of the battery pack by a battery charger will take at least an hour for quick charge and up to several hours for normal charge, which is time consuming. A spare battery pack may be used in the meantime, but it can be an expensive investment and is not environment friendly. One or more fully recharged batteries may be prepared for future use, but they have a tendency to leak or self-discharge when stored for a prolonged period of time.
The subject invention seeks to mitigate or at least alleviate such problems by providing a power supply module for supplying power to an electrical power tool in conjunction with a rechargeable battery pack therefor.
According to the invention, there is provided a power supply module for use between an AC/DC power supply unit and an electrical power tool driven by an electric motor and powered by a rechargeable battery pack that has a normal operating voltage and a relatively higher fully-charged terminal voltage. The module comprises an input connectable to the power supply unit and an output connectable in parallel to the battery pack, and an operating circuitry connected between the input and the output for delivering DC power from the power supply unit to the power tool in parallel with the battery pack supplying DC power to the power tool. The operating circuitry provides a DC operating voltage at the output of a value between the normal operating voltage and the fully-charged terminal voltage of the battery pack, and provides an output current limited to a predetermined magnitude. The operating circuitry comprises a current comparator for determining whether the output current is less than or has a tendency exceeding the predetermined magnitude and then providing a corresponding control signal. The operating circuitry includes a switching device connected at the output for controlling, in response to the control signal, the power delivered to the power tool in an interactive manner as a direct DC current when the output current is below the predetermined magnitude or as a pulsating DC current when the output current has a tendency exceeding the predetermined magnitude.
Preferably, the operating circuitry includes a current sensor connected between the input and the current comparator for sensing the current drawn into and output by the module and then sending a control signal indicative of the magnitude of the output current to the current comparator, and the current comparator is for subsequently comparing the control signal with a predetermined value that represents the predetermined magnitude.
More preferably, the operating circuitry includes an oscillator connected between the current comparator for receiving the control signal therefrom and the switching device for providing a trigger signal thereto for controlling, in response to the control signal, the power delivered to said power tool in the interactive manner.
Further more preferably, the trigger signal provided by the oscillator is of a frequency below 1 kHz.
Further more preferably, the current comparator is arranged also to determine the rate at which the output current tends to exceed the predetermined magnitude and, in response, to control the oscillator to provide a trigger signal for switching the switching device at a frequency that increases or decreases with the rate of increase of the output current.
It is preferred that the operating circuitry provides a substantially constant DC operating voltage at the output.
It is preferred that the DC operating voltage provided by the operating circuitry is 25% higher than the normal operating voltage of said battery pack.
It is further preferred that the DC operating voltage provided by the operating circuitry is substantially 10% higher than the normal operating voltage of said battery pack.
In a preferred embodiment, the current comparator is arranged such that its control signal controls the switching device to deliver power to said power tool as a pulsating DC current whose peak magnitude is maintained substantially at the predetermined magnitude.
Advantageously, the operating circuitry includes a delay element to delay the control signal turning on the switching device in each cycle while delivering power to said power tool as a pulsating DC current.
It is preferred that the current comparator is arranged such that its control signal turns on the switching device continuously that in turn delivers substantially the whole of the current supplied by said power supply unit to said power tool as a direct DC current via a conducting path.
The invention also provides an electrical power tool driven by an electric motor and powered by a rechargeable battery pack, incorporating the aforesaid power supply module. The power tool includes a trigger control circuit connected between the battery pack and the motor, which incorporates a trigger for switching on and off and controlling the speed of the motor. The module is connected at its output in parallel with the battery pack and acts as an auxiliary power source to assist the battery pack.
The invention also provides an AC/DC power supply unit incorporating the aforesaid power supply module. The power supply unit is an AC-to-DC switching mode power supply having an output connected to the input of the module for supplying a DC power to the module. The power supply unit and the module have substantially the same operating voltage.